1. Field of the Invention
The invention relates to an insulating tape which is composed of one or more fabric layers and mica layers and in which the interspaces of the fabric, which is made up of threads, are essentially filled with a resin which is provided with a filler containing microscopically small particles having a thermal conductivity which is greater than that of the resin.
The invention furthermore relates to a method of producing such an insulating tape.
2. Discussion of Background
Large electrical machines having indirectly cooled stator winding impose high requirements on the temperature stability of the winding insulation. Because the heat produced in the conductor copper has to be removed via the main insulation to the (cooled) stator sheet-metal body, said insulation cannot be made indefinitely thick, but, on the other hand, a certain minimum thickness of the main insulation has to be maintained in the case of high-voltage machines with rated voltages of 16 kV and over. There is therefore a great need for high-voltage-resistant insulation materials which have at the same time good thermal conductivity.
The main insulation of the stator winding according to U.S. Pat. No. 4,806,806 or the substantively identical EP-A-0 266 602 comprises a multilayer insulating tape which is wound in many layers around the conductor assembly. The insulating tape is either provided with impregnating resin beforehand or the latter is applied only after the insulating tape has been wound on. The insulating tape itself is composed of a plurality of thin mica layers and reinforcing glass-fabric layers. To improve the thermal conductivity, an inorganic filler having good thermal conductivity, for example boron nitride, aluminum nitride, silicon nitride, aluminum oxide, magnesium oxide, beryllium oxide or silicon carbide, having a particle size of 0.1-15 .mu.m is added to the impregnating resin, the filler content being at least 90% by weight in the interspaces of the glass fabric.
In U.S. Pat. No. 4,806,806 it is pointed out that it is ostensibly of great importance to work with small particle sizes because, on the one hand, the homogeneity of the mixing is thereby improved and, on the other hand, the risk of damage to the sensitive mica layer is ostensibly reduced. The inventors of said patent specification have, however, ignored the fact that the particles of the filler also, and to a particular extent, mechanically attack the fabric threads of the glass fabric. Said fabric threads are composed of fine to superfine glass fibers which are twisted together, the fiber diameter being of the order of magnitude of the particle size of the filler. These sharp-edged particles may damage the fabric threads even during winding and even more so during the subsequent pressing operations, with the result that the glass fabric can no longer, or only incompletely, fulfil its intended purpose of being support and reinforcement for the mica.